1. Field of the Invention
The present invention relates to an image pickup apparatus and method. More particularly, the present invention relates to an image pickup apparatus and method in which influence of a smear phenomenon can be suppressed by smear correction.
2. Description Related to the Prior Art
A digital still camera includes a CCD image sensor (charge coupled device) as image pickup device. In use of the digital still camera, a smear phenomenon is likely to occur. The smear phenomenon is a chroma clip of highlight in a vertically (or horizontally) extending line shape, and occurs specially when an extremely bright portion is included in an object in comparison with a peripheral region.
A smear phenomenon occurs when charge derived from light leaking in an originally shielded peripheral region (for example, vertical transfer path) other than photo diodes comes to pass in a transfer path of original charge of a signal from the photo diodes, or when charge generated in a semiconductor substrate comes to pass in the transfer path. Occurrence of the smear phenomenon is characteristically derived from the structure of the CCD image sensor.
It is known in the art that a smear amount of the smear phenomenon does not depend on an electronic shutter speed of the CCD image sensor but on the intensity of incident light owing to the feature of the smear phenomenon. It is possible to reduce the smear amount remarkably by eliminating unwanted charge passing into the transfer path after the use of a mechanical shutter to shield light to the CCD image sensor.
However, a smear amount increases when a motion image is picked up or a live image is picked up and monitored, as no mechanical shutter can operate. Also, a signal level derived from the smear phenomenon may become considerably high in the image pickup with a high electronic shutter speed.
Various techniques for reducing the smear amount are known. Modification of the CCD image sensor itself for suppressing the smear phenomenon requires very complicated structures. Also, a method of utilizing a signal from a black pixel area (optical black region) outside an active pixel area of the CCD image sensor is known for estimation of occurrence of the smear phenomenon. The smear amount is reduced by subtracting a component from an image signal, the component being derived from the smear phenomenon. However, there is a problem in that influence of the smear phenomenon remains even after the correction, because the subtraction is carried out also for pixels with saturation of charge free from the smear phenomenon, cause gray coloring of an originally white image portion.
A digital still camera of a widely available type has an adjuster for automatically adjusting white balance by estimating color temperature of a light source. The color temperature of the light source is retrieved by evaluating an image signal output by the CCD image sensor. When the smear phenomenon occurs, the color temperature of the light source cannot be obtained correctly. The adjustment of the white balance is likely to create an image without acceptable color balance.
There are known methods of signal processing for correcting the white balance by exact estimation of the color temperature of the light source even in occurrence of the smear phenomenon, for example, JP-A 2002-084546, JP-A 2002-271802 and U.S. Pat. No. 7,358,993 (corresponding to JP-A 2004-015653).
The adjustment of the white balance is based on addition or subtraction of an input gain (white balance gain) with signals of R, G and B color components, the input gain being determined according to the color temperature of the light source being estimated. In contrast, a component of the smear is superimposed to the R, G and B color components of the image signal in an offset manner or equally between the three colors.
There arises a problem in that the smear amount itself cannot be reduced in one image portion even though the white balance is suitably adjusted according to the color temperature of the light source. If the white balance is adjusted with occurrence of the smear phenomenon, coloring occurs on the smear phenomenon due to the calculation with the white balance gain for the smeared portion. The smeared portion is colored so unnaturally by the automatic correction of the white balance that image quality will be low.